Receiver input circuit



May 24, 1949. H.v BLQ-K 2,470,882

RECEIVER INPUT CIRCUII Filed Juh' 2, 1946 'Lr T FIG. I

3 Y V f FIG. 2

INVENTOR HENRI BLOK Patented May 24, 1949 RECEIVER INPUT CIRCUIT Henri Blok, Eindhoven, Netherlands, assignor to Hartford National Bank and Ernst Company, Hartford, Conn, as trustee Application July 2, 1946, Serial No. 680,888 In the Netherlands March 10, 1943 Section 1, Public Law .690, August 8, 1946 Patent expires March 10, 1963 (Ci. 25(i-40) 2 Claims. 1

The invention relates to radio-receivers whose input oscillatory circuit, which may be tuned by varying the self-induction of a tuning coil, is cou pled to the aerial circuit by a capacitative current coupling with the aid of a coupling condenser which has connected in parallel with it an additional coil whose self-induction is a multiple of the maximum self-induction of the tuning coil.

In the usually employed receivers with condenser tuning the aerial circuit is inductively coupled as a rule to the self-inductance coil of the input oscillatory circuit.

This inductive aerial coupling is not suitable, however, for receivers which may be tuned by varying the self-inductance, due to the fact that in this case the coupling would be execessively dependent upon the tuning frequency. In receivers of the last-mentioned type use is therefore frequently made of a capacitative aerial coupling, and preferably of a so-calied current coupling, in which event a coupling condenser incorporated in the aerial circuit forms at the same time part of the input oscillatory circuit.

This capacitative current coupling exhibits, however, the drawback that the coupling condenser constitutes a comparatively high impedance for the mains frequency and for harmonics thereof so that hum voltages which are capacitatively transmitted to the aerial by parts of the receiver which are under tension or by the mains to the aerial, bring about an appreciable voltage on the grid of the first tube. In order to remove this drawback, it has been propcsed to connect in parallel. with the coupling condenser an additional coil whose self-induction is a few times, for example 5 times, as large as the maximum self-induction of the tuning coil. This additional coil practically constitutes a short-circuit for the mains frequency and for the relevant harmonies thereof but it has practically no influence on the action of the coupling condenser for the oscillations to be received. The invention is based upon the recognition that the presence of this additional coil may be utilized to obtain an appreciable improvement in the reception without the use of additional circuit elements, for receivers of the above-described type have the drawback that the transmission of the oscillations received by the aerial to the control grid of the first tube greatly decreases when the frequency increases. This phenomenon is chiefly due to the fact that the damping of the input oscillatory circuit increases with the frequency. Thus, for example, for the lowest frequencies of the medium-wave range the ratio between the voltage on the control grid of the first tube and the aerial electromotive force is approximately 1.5 times as large as for the highest frequencies of this range.

According to the invention, this drawback is removed by inductively coupling the additional coil to the tuning coil and by making this coupling dependent upon the self-induction of the tuning coil in such manner that the transmission of the received oscillations to the control grid of the first tube is constant or at least approximately constant over the whole of the frequency range commanded by the tuning coil.

If the variation of the self-induction of the tuning coil is effected by displacement of a ferromagnetic core, the latter is preferably also led through the field of the additional coil, and this in such manner that the core enters into the additional coil according as it leaves the tuning coil. It is thus achieved that the coupling between the tuning coil and the additional coil increases according as the self-induction of the tuning coil decreases, so that for the highest frequencies of the tuning range an additional coupling is obtained.

The invention will be explained more fully with reference to the accompanying drawing, which represents, by way of example, one embodiment thereof.

Fig. 1 represents part of a radio-receiver wherein an aerial l is connected to earth through the intermediary of a coupling condenser 2 which forms, jointly with a tuning coil 3 and with a fixed condenser 4, an input oscillatory circuit which is incorporated in the control grid circuit of a high-frequency amplifying tube or frequency changer 5.

In order to avoid the production of hum voltages on the control grid of the tube 5, the coupling condenser 2 has connected in parallel with it an additional coil 5 which forms a short-circuit for the hum frequencies. The self-induction of this additional coil is a multiple of the maximum self-induction of the tuning coil 3.

According to the invention, we obtain a uniform transmission of the received oscillations by inductively coupling the additional coil 6 in a suitable manner to the tuning coil 3. One example of the manner in which this coupling may be realized is shown in Fig. 2.

With the device according to Fig. 2 the tuning is effected by displacing a ferromagnetic core I in the field of the tuning coil 3. The core T also passes through the field of the additional coil 6, and this in such manner that upon leaving the coil 3 the core enters into the coil 6. The more the self-induction of the tuning coil decreases the tigher consequently becomes the coupling between the coils 3 and 6, so that at the highest frequencies"of theituning range "an additional coupling is obtained.

What I claim is:

1. A receiver input circuit arrangement to reduce interference from power line frequencies, comprising a first variable inductive elementhaving a given range of self-inductance, a capacitive element in series with said first'variable inductive element, an antenna, means to couple said antenna to the series connection of said first variable inductive element and capacitive element, a second variable inductive element connected in parallel to said capacitive element having a range of self-inductance greater than the first variable inductive element and mutually coupled thereto, and mechanical means to vary simultaneously the mutual inductive coupling between the first and second inductive element and the self-inductance of the firstinductive element.

Y 2. A receiver input circuit arrangement to reduce interference from power line frequencies, comprising a first va'f able inductive elment hav- 4 ing a given range of self-inductance, a capacitive element in series with said first variable inductive element, an antenna, means to couple said antenna to the series connection of said first variable inductive felement" 'a'1 '1"d' capacitive element, a second variable inductive'element connected in parallel to said capacitive element having a range of self-inductance greater than the first variable inductive member and mutually coupled thereto, and means to vary simultaneously the mutual inductive coupling between the first and second inductive element and the self-inductance of the first inductive element, said latter means comprising a ferromagnetic core common to the first and second variable inductive element and axially movable therein.

' HENRI BLOK.

REFERENCES CITED Name Date Beers Aug. 21, 1945 Number 

